2o6 THE FROG chap. 



a linear descendant, through repeated fission, of the oosperm ; 

 and the oosperm itself is the product of the fusion of two 

 cells, one — the ovum — derived from the female, the other 

 — the sperm — from the male parent. Thus in passing from 

 the oosperm to the adult animal there is a gradual structural 

 or morphological differentiation accompanied by a differen- 

 tiation of function or division of physiological labour. 



The expression "division of physiological labour" was 

 invented by the great French physiologist, Henri Milne- 

 Edwards, to express the fact that a sort of rough correspon- 

 dence exists between lowly and highly organised animals 

 and plants on the one hand, and lowly and highly organised 

 human societies on the other. In primitive communities 

 there is little or no division of labour : every man is his own 

 butcher, baker, soldier, doctor, etc., there is no distinction 

 between " classes '' and " masses," and each individual is to 

 a great extent independent of all the rest. Whereas in 

 complex civilised communities society is differentiated into 

 politicians, soldiers, professional men, mechanics, labourers, 

 and so on, each class being to a great extent dependent on 

 every other. 



Structure and Metamorphosis of the Tadpole —Develop- 

 ment of other Organs— Metamorphosis. — A sketch of the 

 further development of the tadpole and of its metamorphosis 

 into the frog has already been given (pp. 9-1 1, Fig. i), and 

 it is now necessary to add a few details. 



A third pair of branchial arches appear behind the two 

 already noticed, and on each a third external gill. The first 

 two pairs increase greatly in size and all the gills become 

 branched (Fig. 66, A). At the same time the branchial clefts 

 come to open into the pharynx, and a current of water 

 enters"the mouth and passes out by the branchial clefts, thus 

 providing the gills with a constant supply of aerated water for 



